The invention relates to rotary drilling tools, for use in drilling subsurface formations, of the kind comprising a tool body having a shank for connection to a drill string, and a plurality of cutters mounted on the tool body and formed with cutting edges defining a cutting profile.
The "cutting profile" of the drilling tool is an imaginary surface of revolution swept out by the cutting edges of the cutters as the tool rotates (with zero rate of penetration).
The invention is particularly, but not exclusively, applicable to drilling tools in which some or all of the cutters are preform (PDC) cutters each formed, at least in part, from polycrystalline diamond. One common form of cutter comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front cutting face of the element, bonded to a substrate which is usually of cemented tungsten carbide.
The tool body may be machined from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in a furnace so as to form a hard matrix.
The invention is particularly applicable to drill bits, and will be particularly described in relation thereto. However, it is to be understood that the invention is also applicable to other forms of drilling tools, such as hole openers and eccentric hole openers.
While PDC bits have been very successful in drilling relatively soft formations, they have been less successful in drilling harder formations and soft formations which include harder occlusions or stringers. Although good rates of penetration are possible in harder formations, the PDC cutters suffer accelerated wear and bit life can be too short to be commercially acceptable.
Studies have suggested that the rapid wear of PDC bits in harder formations is due to chipping of the cutters as a result of impact loads caused by vibration, and that the most harmful vibrations can be attributed to a phenomenon called "bit whirl". Bit whirl arises when the instantaneous axis of rotation of the bit precesses around the central axis of the hole when the diameter of the hole becomes slightly larger than the diameter of the bit. When a bit begins to whirl some cutters can be moving sideways or backwards relatively to the formation and may be moving at much greater velocity than if the bit were rotating truly. Once bit whirl has been initiated, it is difficult to stop since the forces resulting from the bit whirl, such as centrifugal forces, tend to reinforce the effect.
One method which has been employed to overcome bit whirl is to design the drill bit so that it has, when rotating, an inherent lateral imbalance force which is relatively constant in direction and magnitude. The gauge of the bit body then includes one or more low friction bearing pads which are so located as to transmit this lateral imbalance force to the part of the formation which the bearing pad is for the time being engaging. The low friction bearing pad thus tends to slide over the surface of the formation which it engages thereby reducing the tendency for bit whirl to be initiated.
In an alternative approach, bits have been designed in a manner to provide a structure which constrains the bit to rotate truly, i.e. with the axis of rotation of the bit coincident with the central axis of the borehole. One such approach is described in a paper titled "A new PDC cutting structure improves bit stabilisation and extends application into harder rock types", Paper No. SPE/IADC 25734 by G. E. Weaver and R. I. Clayton, Society of Petroleum Engineers, SPE/IADC Drilling Conference, Amsterdam, 23-25 Feb. 1993.
In PDC bits the cutters are normally arranged in spiral arrays with respect to the central axis of rotation of the bit so that the path swept by each cutter during each rotation overlaps the paths swept by other cutters disposed at slightly greater and slightly smaller radial distances from the bit axis. This provides an essentially smooth cutting profile to ensure that no part of the formation at the bottom of the borehole remains uncut. By contrast Weaver and Clayton proposed a cutter formation where the cutters, instead of being located in spiral formations, are disposed in concentric radially spaced arrays centred on the axis of rotation of the bit. In such an arrangement the cutters in each circular array sweep through essentially the same cutter path and the cutter paths of adjacent arrays do not overlap but are spaced apart in the radial direction. Consequently, the cutters define a series of concentric annular grooves in the cutting profile. As a result the cutters in each circular array cut a deep groove in the formation at the bottom of the borehole with annular ridges of uncut formation extending upwardly between the adjacent circular arrays of cutters.
The annular ridges increase significantly the vertical contact between the cutters and the formation so that any lateral force acting on the bit, whether externally generated or from cutting structure imbalance, is distributed over a larger contact area. This reduces the unit stress on the formation and the result of lower unit stress is said to result in less tendency for a cutter to bite laterally into the formation and initiate bit whirl.
However, such arrangements depend, in operation, on the upstanding annular ridges of formation between the cutter arrays eventually breaking off when they reach such a height that they cannot withstand even the lower unit lateral stress applied to them. In order to ensure that this occurs, it is necessary for the annular ridges to be of narrow radial width. It also means that the point at which an annular ridge breaks off may be unpredictable since it will depend on the nature of the formation in the ridge and the lateral force which happens to be applied to the ridge during drilling. If a ridge breaks off when it is comparatively low in height, it will not provide a useful contribution to inhibiting lateral displacement of the bit and inhibiting the initiation of bit whirl. On the other hand, if a ridge does not break off until it has reached a considerable axial depth, the ridge may bear on and abrade the surface of the bit body between the adjacent arrays of cutters, resulting in wear of the bit body, an increase in the frictional restraint to rotation of the bit, and the necessity of increasing the weight-on-bit in order to continue drilling at the same rate of penetration.
International Patent Application No. WO 93/13290 (Dresser Industries Inc.) describes various drill bits of the above-mentioned kind, including arrangements where means are provided to assist removal of the tops of the ridges as drilling progresses.
The present invention relates to improved designs of rotary drill bit in which the above-mentioned disadvantages may be overcome.